In a light valve system, it is especially important to maximize the collection of light available from the source to maximize efficiency of the system and maintain a cost effective design. The cost of precision light sources such as for display systems increases dramatically with lumen intensity. Light valve systems can be categorized into transmissive and reflective systems.
In a typical transmissive system, a collecting mirror is used to collect light and transmit it through the valve. With only a collecting reflector, light is collected from only one side of the source. Light collected from collecting reflectors has a low degree of collimation. Nevertheless, a typical transmissive light valve system collects light with collecting reflectors. Such transmissive systems can collect over 55% of the light radiated from the source.
For reflective light valve systems, the requirements for light collimation are more stringent and typical systems use condensing lenses to collect light which give much better collimation. A typical reflective light valve system uses both a spherical mirror and a condensing lens or lens system so that light is collected from 2 sides of the source. However, the efficiencies of such systems are low. For example, an F/0.72 condensing system with a rear reflector collects only 18% of radiated light.
FIG. 1 demonstrates a typical diffractive light valve configuration where the source is imaged on a turning mirror. Light is radiated in all directions from a light source 100, such as a bulb. Light which strikes a spherical mirror 102 is collected and returned to the source 100 and passes thereby. Similarly, light which leaves the source 100 and strikes a condenser system 104 is also collected and focused. In the embodiment shown in FIG. 1, the condenser 104 is a composite lens system. It will be apparent to those of ordinary skill in the art that other types of lens systems can be utilized to accomplish the function of condensing. The light which was collected by the mirror 102 also enters the condenser 104 after passing the source 100. Light which neither strikes the mirror 102 nor the condenser 104 is lost. This wasted illumination accounts for lost efficiency of the system.
The condenser 104 images the light from the source 100 onto a specular turning mirror 106. The turning mirror 106 reflects the light onto a reflective light valve 108. In the embodiment shown in FIG. 1, a Schlieren lens 110 is positioned between the turning mirror 106 and the valve 108.
The valve 108 in this embodiment is a diffraction grating light valve. The valve is selectively configurable to be either reflective or diffractive. Examples of such a device are found in U.S. Pat. No. 5,311,360, issued May 10, 1994 to Bloom et al., and in co-filed, co-pending U.S. patent applications, Ser. No. 08/482,188, entitled FLAT DIFFRACTION GRATING LIGHT VALVE, and filed herewith, and in co-filed, co-pending U.S. patent application, Ser. No. 08/480,459, entitled A METHOD OF MAKING AND AN APPARATUS FOR A FLAT DIFFRACTION GRATING LIGHT VALVE and filed herewith. When configured to be reflective, the light which strikes the valve 108 is specularly reflected back to the turning mirror 106 through the lens 110 and thence to the source through the condenser 104.
When the valve is diffractive, the diffractive angle is known and calculable as a function of the period of the diffraction grating. When configured to be diffractive, the system is configured so that the deflected light bypasses the turning mirror and enters the projection optics 112. It is well known that light is diffracted through first order angle .THETA. and also through multiples of .THETA.. Light which leaves the valve 108 at the second and third order diffraction angles blocked by the pupil mask 114. In this way, the desired first order light is provided to the projection optics 112.
It is required that the deflection angle of the light valve be greater than the angle subtended by the image of the source on the turning mirror to collect all of the deflected light. For large sources, this is not always possible without compromising brightness. This constrains the maximum magnification of the source on the turning mirror or requires alternative techniques to maximize brightness. These problems become pronounced for light valves that deflect light through small angles.
The Eidaphor is the oldest high-brightness reflective light valve projector. The Eidaphor utilizes a deformable oil film on a spherical mirror that is addressed with an electron beam. When addressed, the locally flat surface of the film becomes sinusoidal with a period of 40 .mu.m. The size of the source incident on the turning mirror must be small. To achieve small turning mirror dimensions while still maximizing brightness, multiple turning mirrors are used. The light is only collected in a single dimension and direction for delivery the multiple turning mirrors. Light from a source is collected in a single dimension by a condenser lens system. The light leaving the lens system in a single direction impinges on multiple turning mirrors. By using multiple turning mirrors, the effective size of the turning mirrors when taken together as viewed from the source is increased. Correspondingly, the smaller turning mirrors allow higher order modes of diffracted light to bypass the mirrors. The light reflected by the turning mirrors passes through a splitter to impinge on the color specific deformable oil film/reflectors. Three oil film/reflectors are used to develop red, green and blue color. A lens system is used between the splitter and the oil film/reflectors. If the light is reflected, it returns to the source. If the light is diffracted, it bypasses the turning mirrors and proceeds to the projection optics. As with the embodiment of FIG. 1, a significant portion of the light generated by the source is wasted and never enters the system or impinges on the valve.
For a diffractive light valve which diffracts a small amount of undesirable light into the second order, this system has the disadvantage that second order light will be diffracted into the dark state. This is an advantage for the Eidaphor, because its dark state does not diffract light into higher orders.